Digital imaging devices are becoming increasingly popular in a variety of applications, including digital cameras, fingerprint recognition, digital scanners and copiers, and the like. Typical prior art digital imaging devices are based on charge coupled device (CCD) technology. CCD devices have an array of CCD cells, each cell comprising a pixel. Each CCD pixel outputs a voltage signal proportionate to the intensity of light impinging upon the cell. This analog voltage signal can be converted to a digital signal for further processing, digital filtering, storage and the like. As is well known in the art, a two dimensional digital image can be constructed from the voltage signals output from a two-dimensional array of CCD cells, commonly referred to as a sensor array.
CCD arrays have a shortcoming in that CCD fabrication requires a special process that is not compatible with standard CMOS processes. Thus, the CCD array cannot be easily integrated with other logic circuits, such as CCD control logic, analog to digital converters, and the like. Additionally, in operation a CCD array requires multiple high voltage supplies from 5V to 12V and CCD arrays tend to consume a large amount of power in use.
An alternative to CCD arrays is using an array formed of CMOS cells. A CMOS sensor array can be fabricated using standard CMOS processing and thus can be integrated onto a single chip with other circuits, such as array control logic, analog to digital converters (A/D's), digital signal processing (DSP) cores, and the like. CMOS arrays provide the additional advantage of operating with a single low supply voltage such as 3.3V or 5V, and consuming less power than a comparable CCD array. Finally, a CMOS array can be fabricated at a lower cost than a similar CCD array.
One common problem with both CCD and with CMOS imagers is that of point defects which cause “spot noise” on the image, such as white spots on a dark portion of the image or a dark spot on a white portion of the image. In CMOS imagers, white spots are due to pixels (i.e. CMOS cells) with excessive leakage current. Dark spots are due to either particles covering the pixel or a defect in the pixel electronics causing the pixel not to turn on. Spot noise seriously limits the yield of CMOS imagers, resulting in increased costs.
One method to remove spot noise electronically has been proposed by Younse et al. in U.S. Pat. No. 4,805,023. The Younse et al. implementation requires expensive EPROM memory and involves a complicated hardware system, further increasing the imager cost. Furthermore, the solution proposed by Younse et al. cannot remove temperature dependent spot noise, such as white spots appearing only at high temperatures.
Therefore, a need exists for a relatively inexpensive defective pixel filter that can quickly and reliably filter out the effects of defective pixel such as spot noise from an image signal.